1. Field of the Invention
The present invention relates to an inorganic fine particle and a method for producing the same. Particularly, the present invention relates to a technique for producing a titanium dioxide fine particle industrially utilized in wide applications such as white pigments, photocatalytic materials, and medical materials.
2. Description of the Related Art
Inorganic fine particles are widely utilized as materials for a variety of industrial products. Particularly, titanium dioxide, one of the inorganic fine particles, is utilized in wide applications such as white pigments for electronic displays, paints, inks, plastics, fibers, cosmetics, and paper and as photocatalytic materials for construction materials by exploiting its light scattering ability or photoactivated decomposition ability (e.g., Japanese Patent Application Laid-Open Nos. 2005-272270, 2006-037090, and 2006-008475). Since titanium dioxide is an exceedingly stable substance in the atmosphere or in solutions and is safe with low toxicity, its use as a medical material such as a drug delivery system (DDS) in cancer therapy and the like is also expected (e.g., Japanese Patent Application Laid-Open No. 2005-289660).
Such industrially useful performance of titanium dioxide fine particles is expressed mainly depending on the size, size distribution, and crystal structure thereof. Particularly, for the expression of optical or photocatalytic properties, it is important to increase the surface area per unit volume. Specifically, it is important to produce fine particles at a nanometer level. For uniformity in the expressed performances, it is important to make fine particle size distribution monodisperse.
The production of favorably monodisperse fine particles is required not only for titanium dioxide but also for many other inorganic materials. Techniques for producing fine particles become increasingly important for the future industrial use of inorganic materials.
General methods for producing fine particles are broadly classified into a breakdown method as shown in, for example, “The 4th Series of Experimental chemistry” (ed., The Chemical Society of Japan, 1993, vol. 12, p. 411-488, Maruzen Co., Ltd.) wherein fine particles are produced from bulk substances by pulverization or the like, and into a buildup method wherein fine particles are produced by particle growth from a gaseous or liquid phase. The pulverization technique using the breakdown method has frequently been used so far and is a highly practical method for producing fine particles. However, this technique presents various problems such as exceedingly low productivity and a limited number of applicable substances in producing particles of nanometer size. Thus, a method for producing fine particles of nanometer size by the buildup method has been studied in recent years.
The buildup methods are broadly classified into solid-phase, gaseous-phase, and liquid-phase methods according to the type of a reaction phase. The gaseous-phase and liquid-phase methods are allegedly suitable for the synthesis of fine particles of nanometer size. The gaseous-phase methods are classified into a physical agglomeration process by high-temperature vapor cooling (physical vapor deposition (PVD) process) and into a particle generation process by gas-phase chemical reaction (chemical vapor deposition (CVD) process). These approaches can produce highly pure particles having a small diameter and as such, have been utilized widely. However, the approaches present problems in strictly controlling particle sizes and monodispersity. On the other hand, the liquid-phase methods have an advantage of being capable of mixing raw materials at a molecular level and are classified into sol-gel, alkoxide, reversed micelle, hot-soap methods, and so on.
One of methods for producing fine particles by the liquid-phase method of the buildup method, which has gotten attention recently, uses a “micro chemical process technique” (e.g., Japanese Patent Application Laid-Open No. 2005-288254). The “micro chemical process technique” is a technique for efficiently performing chemical reaction by use of a reaction channel having a very small cross section. Specifically, this technique performs substance production, chemical analysis, and so on, by utilizing chemical/physical phenomena occurring in a microchannel of several μm to several hundreds of μm in width formed on a solid substrate by a microprocessing technique or the like. M. Takagi et al. (“Production of titania nanoparticles by using a new microreactor assembled with same axle dual pipe”, Elsevier B. V., Chem. Eng. J. 2004, 101, p. 269-276) and T. Tsujiuchi et al. (“Study on control method for particle growth process using double-pipe microreactor”, Proceedings of the 37th Autumn Meeting of The Society of Chemical Engineers, Japan, I-317, 2005) have proposed the production of titanium dioxide fine particles using, as a mixing propulsion, molecular diffusion that utilizes orderly laminar flows in a microchannel, wherein the average size of the titanium dioxide fine particles is controlled within a range of 50 to 80 nm, with their monodispersity maintained, by changing operation conditions such as the diameter of a double-pipe microchannel and a flow rate.